This invention has to do with automatic frequency control systems. More particularly, it concerns systems and methods for adjusting the frequency of an alternating signal using a phase-lock-loop wherein the frequency is adjusted to track a frequency corresponding to a particular characteristic of an input signal device.
To achieve good noise rejection in circuit applications, as for example in the input to communication receivers, it is desirable to match the bandwidth limiting characteristics of the input device to the spectral characteristics of the input signal. Where a frequency misalignment of one characteristic relative to the other occurs, either because of circuit drift, signal shift, or design tolerance, typically, either signal power is lost or signal distortion is introduced. Therefore, it is desirable to be able to adjust one spectral characteristic relative to the other and bring them into alignment.
For an initial misalignment resulting from design tolerances, alignment is possible by manually trimming either the spectral characteristics of the input signal or the passband characteristics of the input device. Manual adjustments may also be used to overcome long term drift arising, for example, from component aging, where intervals between adjustment are acceptable. However, where the rate of drift or frequency shift is substantial relative to the criticality of spectral alignment, automatic methods of frequency alignment or synchronization must be employed.
The literature is replete with descriptions of systems and methods for automatic frequency control. In particular, those concepts which are categorized under the popular term "phase-lock-loop" with which the present invention is closely related, are representively described in the following book publications. They are: Phaselock Techniques by Floyd M. Gardner, Second Edition, John Wiley and Sons, 1979; Principles of Coherent Communication by Andrew J. Viterbi, McGraw-Hill, Inc., 1966; Synchronization Systems in Communication and Control by William C. Lindsey, Prentice-Hall, Inc., 1972; and Phase-Locked Loops and Their Application, edited by William C. Lindsey and Marvin K. Simon, IEEE Press, 1978.
Despite the abundance of concepts described in the aforementioned publications, there appears to be an absence of a concept or system for automatically adjusting the frequency of an alternating signal, such as an oscillator output, to minimize the spectral misalignment between the input signal spectrum, which in some manner is coherently derived from the alternating signal, and the band-limiting characteristics of the input device, wherein the spectral misalignment results from either design tolerances or the frequency drift in the band-limiting input device.